1. Field of the Invention
The invention relates to a communication system, and more particularly, to a multi sub-carrier communication system for receiving and transmitting a signal encoded based on a quadrature amplitude modulation (QAM).
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) systems use multiple sub-carrier frequencies (sub-channels) to transmit data.
In a multi sub-carrier communication system, a frequency equalizer (FEQ) is generally realized by calculating a channel distortion value H based on an average value of a predetermined number of received signals during estimating the channel and then taking an inverse value H−1 of the calculated channel distortion value to determine equalization coefficients (an equalization vector) of a frequency equalizer. The performance level of the FEQ is dependent on the number of samples taken from the received signals.
Typical protocols of a wired multi- sub-carrier communication system perform a channel analysis within a limited time (during an initialization interval) to determine a FEQ coefficient (vector). Therefore, when there are many noises in a received signal, or when the number of received signals is small, the performance of the frequency equalizer may be degraded. Even if the performance of the frequency equalizer is optimized luckily by accurately estimating a channel characteristic, there still remains a chance that the performance of the frequency equalizer may be diminished by variations of the channel characteristics after a predetermined time has elapsed.
The channel characteristic of a wired channel such as a telephone line rarely changes within a short period of time but the channel characteristic may vary over a long period of time. Thus, it is necessary to repeatedly optimize the FEQ coefficient vector corresponding to the channel characteristic change. However, it is difficult to estimate an accurate channel characteristic for each time interval. In case where a high level of noise exists among received signals used for estimating the channel characteristic, or the number of samples is small, the performance of the frequency equalizer may be significantly degraded.